1. Field of the Invention
The present invention relates to a steering control apparatus for an automotive vehicle capable of steering the vehicle automatically without driver's manual operation of a steering wheel.
2. Discussion of the Related Art
There has been developed and placed on the market in recent years an automotive vehicle wherein a steering control apparatus is changeable between steering the vehicle in a manual steering mode and in an automatic steering mode. In the automatic steering mode, for example, in an automatic garaging mode, the vehicle can be conveniently steered automatically to be parked in a garage. In the concrete, the vehicle is stopped at a starting position close to the garage, and the automatic steering mode is activated, wherein a target position in the garage and a path along which the vehicle runs from the starting position to the target position are estimated, and a steered angle of steered wheels (e.g., front wheels of the vehicle) at each position along the path is calculated. Thereafter, when a driver operates an accelerator and a brake pedal, the vehicle runs with the steered wheels being automatically steered so that deviation of the steered angle of the steered wheels detected by a sensor at each position of the path from the calculated steered angle becomes zero, and thereby even an unskilled driver can easily park the vehicle in the garage as described in Japanese unexamined, published patent application No. 2004-42769.
By the way, the steering control apparatus generally substitutes a rotational angle of a steering wheel for the detected steering angle of the steered wheels in executing a positioning control of the steering angle of the steered wheels. And, the steering control apparatus in the prior art receives the rotational angle of the steering wheel, that is, the detected steering angle, from a steering wheel rotational angle sensor, and calculates an electric current command value for driving an electric motor so as to coincide the detected steering angle with the target steering angle of the steered wheels that is estimated according to the target position to which the vehicle is to run. The electric motor is connected between the steering wheel and the steered wheels through a reduction mechanism.
However, the resolution of the steering wheel rotational angle sensor is too low for the prior automatic steering apparatus to execute the stable positioning control. That is, even though the rotational angle of the steering wheel actually changes continuously, the steering control apparatus receives signals indicating the rotational angle of the steering wheel, which changes intermittently because the resolution of the steering wheel rotational angle sensor is low. It causes the output of the steering control apparatus to oscillate or causes large response delay thereof, and thereby it occasionally becomes difficult for the steering control apparatus to stably control the steering angle of the steered wheels under the positioning control. Simply using a steering wheel rotational angle sensor with high resolution to solve such problem causes another problem that the size of the steering control apparatus becomes large and the cost for it rises.